High pressure saturation vapor sodium lamp containing mercury



K. SCHMIDT May 21, 1968 HIGH PRESSURE SATURATED VAPOR SODIUM LAMP CONTAINING MERCURY 2 Sheets-Sheet Filed April 26, 1966 \E Q m mmmmm 0.6 0.4 HTOM FE/QCT/ON 0F SOD/UM IN EXCESS LIQUID l/npo/e PEESSUEES 0F Na AW!) Hg .47 700 C #75 A Fu/vcT/oN 0F COMPOS/T/ON.

i 9 e i m n V A .mwfi K H Unite States Patent 3,384,798 HIGH PRESSURE SATURATION VAPOR SODIUM LAMP CONTAINING MERCURY Kurt Schmidt, Cleveland Heights, Ohio, assignor to general Electric Company, a corporation of New ork Filed Apr. 26, 1966, Ser. No. 545,449 Claims. (Cl. 313184) This invention relates to high pressure sodium vapor lamps of the generic kind described in my copending application Ser. No. 263,676, filed Mar. 1, 1963, now US. Patent No. 3,248,590 entitled, High Pressure Sodium Vapor Lamp, and assigned to the same assignee as the present invention. The present invention is concerned with improvement achieved by providing mercury along with sodium in the filling, the mercury to serve as a buffer gas for the sodium.

As described in my aforementioned application, the high pressure sodium vapor lamp utilizes a slender tubular envelope of material able to withstand the attack of sodium vapor at high temperatures, a suitable material being transparent high density polycrystalline alumina. The are tube is provided with a filling of sodium and an inert gas such as xenon. With sodium vapor pressure in the range of 30 to 1000 torr, a large percentage of the total radiation is emitted on either side of the yellow resonance D lines of sodium by reason of imprisonment and self-reversal of the line radiation. As a result, the radiation is not the monochromatic yellow color of the low-pressure sodium lamp but is golden white with a relatively large amount of energy in the red. The function of the xenon is to act as a starter gas for the lamp.

In one form of lamp, mercury is added to the filling. The function of the mercury is to serve as a buffer gas and produce the proper temperature distribution in the plasma and at the envelope walls. By so doing, the voltage gradient of the arc is increased resulting in a lamp operating at a higher voltage and lower current. In practice, this permits higher efficacies in the current range of 2.5 to 5.0 amperes in which high pressure mercury vapor lamps in the common sizes have traditionally operated, and at the same time it permits savings in ballast costs. Even though the partial pressure of mercury in the lamp may be several times greater than that of sodium, little radiation of mercury lines is apparent in the visible spectrum and it is primarily the sodium atoms that are excited and produce light.

The object of the present invention is to provide further improvements in high pressure sodium vapor lamps in respect to efficiency through the provision of a controlled amount of mercury.

In my earlier copending application, in order to obtain the improvements in efficiency from having mercury vapor present in a high pressure sodium vapor lamp, I proposed one form of lamp having an excess amount of sodium but a limited amount of mercury intended to be completely vaporized during operation. As a result of extensive testing of sodium lamps with various additions of mercury, I have now determined that it is not possible to have a limited amount of mercury in vapor phase over an excess liquid pool of sodium. What happens is that the sodium and mercury form an amalgam and the vapor pressure of both constituents over the amalgam for a given temperature is reduced. This means that whenever there is an excess liquid pool of either sodium or mercury, one cannot have limited amount or unsaturated vapor operation of the other vapor component. In other words one species cannot be completely vaporized while there remains an excess of the other in liquid form. The thermodynamic reason for this is that the change in free 3,384,798 Patented May 21, 1968 energy of mixing with composition for the liquid at infinite dilution is extremely large.

A problem encountered with the high pressure sodium vapor lamp utilizing an alumina ceramic envelope in its present state of developement is a gradual clean-up of the sodium over the life of the lamp. Therefore a practical lamp requires an excess of sodium in order to avoid sodium starvation before the projected end of life, and this means that the sodium vapor must exist over excess liquid.

In accordance with the invention, in order to have a predetermined pressure of both sodium and mercury under the foregoing circumstances, an excess of both sodium and mercury is provided, that is, substantially more of both than is vapourized in operation. The sodium and mercury may be dosed into the lamp in the form of an amalgam, but if dosed in separately, will form an amalgam on contact within the lamp envelope. For convenience these two constituents of the filling will henceforth be referred to as the amalgam. In order to have acceptably constant pressures of sodium and mercury vapors throughout the projected life of the lamp, I have determined that an excess of amalgam is desirable wherein the atom fraction of sodium to total sodium'and mercury atoms is from 0.5 to 0.9. By this means partial pressures of sodium in the range of 30 to 1000 torr may be achieved simultaneously with partial pressures of mercury in the range of 0.1 to 5 atmospheres, over an excess pool of amalgam at a temperature in the range of 600 to 950 C. as determined by the coldest spot in the lamp envelope. Within these ranges of parameters, maximum efficiency and optimum color rendition are found for practical high pressure sodium vapor lamps.

For further objects and advantages and for a better understanding of the invention, attention is now directed to the following description of a preferred embodiment to be taken in conjunction with the accompanying drawing. The features of the invention believed to be novel will be more particularly pointed out in the appended claims.

In the drawing:

FIG. 1 illustrates a jacketed high pressure sodium vapor lamp embodying the invention.

FIG. 2 is a graph illustrating the vapor pressure characteristics of sodium-mercury amalgam at one temperature.

Referring to FIG. 1, the illustrated high pressure sodium vapor lamp 1 comprises an outer vitreous envelope or jacket 2 of elongated ovoid shape. The neck 3 of the jacket is closed by a re-entrant stem 4 having a press 5 through which extend relatively stilt inlead wires 6, 7 connected at their other ends to the threaded shell 8 and center contact 9 of a conventional screw base.

The inner envelope or are tube 11 which forms the discharge lamp proper is made of sintered high density polycrystalline alumina ceramic or other light-transmitting material capable of withstanding the attack of sodium vapor at high tempertures. A suitable material is disclosed in US. Patent No. 3,026,210Coble, Transparent Alumina and Method of Preparation. The ends of the tube are closed by thimble-like niobium metal end caps 12, 13 hermetically sealed to the alumina by means of a sealing composition or glass comprising primarily aluminum oxide and calcium oxide. The tube is shown sectioned at lower end cap 12 and the sealing glass, greatly exaggerated in thickness in order to permit illustration, is indicated at 14. A pair of thermionic electrodes 15, 15 are mounted in the ends of the arc tube. Each electrode consists of a double wound tungsten wire coil with the interstices filled with activating material in the form of.

alkaline earth oxides including barium oxide. The tungsten coil is wound over a tungsten shank 16, which is crimped in the end of metal tube 17 joined to the end cap. Lower metal tube 17 is pierced through at 18 and used as an exhaust tube during manufacture and for the purpose of introducing the gas filling and sodium mercury amalgam dose into the arc tube. It is hermetically pinched off by a cold weld indicated at 19. Upper metal tube 17 is blocked off internally and need not be pinched.

The are tube is supported Within the outer envelope by means of a single side rod frame 21 which extends the length of the envelope from inlead 7 at the stem end to a dimple 22 at the dome end to which it is anchored by a resilient clamp 23. End cap 13 of the arc tube is connected to the frame by weld junctions to a wire strap 24, While end cap 12 is connected to inlead 6 through band 25 and connecting rod 26. The interenvelope space is desirably evacuated. in order to conserve heat; this is done prior to sealing off and a getter, suitably barium metal powder pressed into the channelled rings 27, is flashed after scaling in order to assure a high vacuum.

Design considerations Many factors influence the choice of geometry and operating point of a high pressure sodium vapor lamp and have to be considered in its design. The wall temperature of the alumina arc tube should not exceed 1400 C. Above this temperature, the rate of evaporation of alumina with subsequent decomposition into aluminum or lower oxides of aluminum and deposition thereof on the inside of the outer envelope where it causes blackening, is excessive. In a 400-watt size of the lamp illustrated in the drawing which is now commercially available, the arc tube is approximately 7.4 millimeters in internal diameter, 9.3 centimeters in length and has a 7 centimeter arc gap between the electrode tips. At the rated wattage, the bulb wall temperature is at a safe 1300" C.

The filling of the arc tube comprises the starting gas and the sodium-mercury amalgam. For efficiency, it is desirable that the starting gas have a low thermal conductivity and for this reason, xenon is preferred as taught in my copending application. In the illustrated lamp, xenon at a pressure of 20 torr is used and starting of the lamp is accomplished by superimposing a high voltage pulse (2500 volts) on the open circuit voltage. It may be noted that Penning mixtures, for instance neon with from 0.1 to 1% argon, can be used as starting gases to bring the starting voltage down to a few hundred volts. However Penning mixtures have high thermal conductivity and cause a serious drop in efficiency, as much as to Amalgam composition The sodium mercury amalgam composition determines for a given control temperature the vapor pressure of sodium and mercury, and thus becomes a primary determinant of lamp voltage, color rendition and efliciency. The variation in partial vapor pressures of sodium and mercury over excess liquid amalgam at 700 C. as a function of composition of the excess liquid is shown in FIG. 2. It will be observed that the partial pressure of sodium is fairly constant for atom fractions above 0.5; below this figure P falls off badly. Since the vapor pressure of sodium is critical in respect of efliciency and color rendition, it is desirable to avoid atom fractions below 0.5. The partial pressure of mercury rises relatively rapidly with the atom fraction. However if the atom fraction of mercury is reduced below 0.1 (corresponding to an atom fraction of sodium greater than 0.9), the vapor pressure of mercury falls off precipitously. Since the mercury vapor pressure is a primary determinant of lamp voltage, atom fractions of mercury below 0.1 should be avoided. There fore it is desirable in accordance with my invention to provide a lamp filling wherein the composition of the excess amalgam in the lamp has an atom fraction of sodium in the range from 0.5 to 0.9. The proportions by Weight will differ of course from the atomic proportions, the weight of each component being in relation to the product of the atom fraction by its atomic Weight. Thus 50 atomic percent sodium corresponds to 10 parts sodium,

parts mercury by weight; 90 atomic percent sodium corresponds to 51 parts sodium and 49 parts mercury by weight.

As previously stated, it is necessary to provide an excess of amalgam in order to compensate for sodium clean-up during lamp life. The excess collects at the coldest place in the lamp envelope and this is generally the lower end cap 12 and the pinched off exhaust tube 17, the latter generally referred to as the appendix. By lengthening the exhaust tube or appendix, a place within the lamp envelope appreciably cooler than the end cap may be provided. I have found that in order to avoid excessive rates of reaction of sodium with the seal material excessively high temperatures, for instance above 800 C., should be avoided. I have found a seal temperature of about 725 C. satisfactory and this can go along with a control temperature at the appendix which is slightly lower, about 700 C.

Under the above-described conditions, I have found that for high efiiciency with acceptable color rendition, an amalgam composition of 70 to 80 atomic percent sodium, preferably approximately 75 atomic percent sodium, is most satisfactory. For the lamp illustrated in FIG. 1 in a 400watt size using an envelope having an internal diameter of 7 to 8 millimeters, suitably 7.4 millimeters, a preferred 75 atomic percent sodium dose consists of about 54 milligrams of amalgam, made up of 14 mg. of sodium and 40 mg. of mercury. This dose produces about 80 torr P and 0.38 atm. P for an appendix control temperature of 700 C. An efiicacy of about lumens per watt is achieved with acceptable color rendition having a red factor of 3; the lamp operates at 4.7 amperes with a voltage drop of 100 volts. The red factor is an arbitrary measure of the proportion of radiation occurring in the visible range above 6000 A.; by comparison, a common high pressure mercury vapor lamp has a red factor of 1, and sunlight varies from 2 to 3 in red factor depending on time, place and weather.

By increasing the mercury content in the above-described dose, somewhat higher efficiencies can be obtained but the color rendition deteriorates and the red factor drops below 3. If a higher percentage of sodium is used, the red factor increases but the efiiciency falls below 100 lumens per watt.

I have found that a lamp having superior color rendition at relatively high efiiciency can be made by reducing the inside diameter of the envelope to about 6 millimeters and using the same sodium-mercury dosage described above. For this lamp, it is desirable to raise the control temperature of the amalgum reservoir to about 800 C. which results in vapor pressures of sodium and mercury of approximately 205 torr and 1.55 atmospheres respectively. The lamp operates approximately at a current of 2.5 amperes with a voltage drop of volts amounting to 400 watts input, a red factor of 5.5 and an eflicacy of 100 lumens per watt.

While the invention has been described by reference to specific preferred embodiments, the details of construction illustrated and described are intended as exemplary and not in order to limit the invention thereto except insofar as included in the accompanying claims.

What I claim as new and desire to secure by Letters Patent of the United States is:

1. A high intensity sodium vapor discharge lamp comprising a tubular elongated envelope of material resistant to the attack of sodium vapor at high temperatures, a pair of electrodes sealed into opposite ends, an ionizable medium within said envelope comprising an inert starting gas and a quantity of a sodium-mercury amalgam in excess of that vaporized in operation of said lamp, the atomic fraction of sodium in the excess amalgam being in the range from 0.5 to 0.9 whereby a partial pressure of sodium may be developed in the range of 30 to 1000 torr along with a partial pressure of mercury in the range of 0.1 to 5 atmospheres when the excess amalgam in said 5 lamp envelope is maintained in liquid form at a temperature in the range of 600 to 950 C. at the coldest place in said lamp envelope.

2. A lamp as defined in claim 1 wherein the atom fraction of sodium in the excess amalgam is in the range of 0.7 to 0.8.

3. A lamp as defined in claim 1 wherein the envelope internal diameter is 7 to 8 millimeters and the atom fraction of sodium in the excess amalgam is about 0.75.

4. A lamp as defined in claim 1 wherein the envelope internal diameter is 7 to 8 millimeters, the atom fraction of sodium in the excess amalgam is about 0.75, and the control temperature is about 700 C. to provide about 80 torr P and 0.38 atmospheres P References Cited UNITED STATES PATENTS 6/1939 Krefft et a1 313221 4/1966 Schmidt 313-184 JAMES W. LAWRENCE, Primary Examiner.

R. JUDD, Assistant Examiner. 

1. A HIGH INTENSITY SODIUM VAPOR DISCHARGE LAMP COMPRISING A TUBULAR ELONGATED ENVELOPE OF MATERIAL RESISTANT TO THE ATTACK OF SODIUM VAPOR AT HIGH TEMPERATURES, A PAIR OF ELECTRODES SEALED INTO OPPOSITE ENDS, AN IONIZABLE MEDIUM WITHIN SIAD ENVELOPE COMPRISING AN INERT STARTING GAS AND A QUANTITY OF A DODIUM-MERCURY AMALGAM IN EXCESS OF THAT VAPORIZED IN OPERATION OF SAID LAMP, THE ATOMIC FRACTION OF SODIUM IN THE EXCESS AMALGAM BEING 